Conventionally, in the case where a special effect image is prepared such that one image that moves while retaining residual images in stereoscopic space intersects another image, it is thought that past images A1, A2 . . . A5 remain at positions at an infinite point (Z=.infin.) in a backward direction (on the Z-axis in other words) with respect to a screen surface 1 (located on the XY-coordinate axis) so that residual images A11, A12, . . . A15 of the past images A1, A2, . . . A5 of a moving body A can be projected on a screen surface 1 to be produced as shown in FIG. 1(A). Out of the past images A1, A2, . . . A5, the priority of an image is higher the nearer it is to the screen surface 1. Thus, priority is set so that higher priority is given to a position located at a place nearer to the screen surface. As a consequence, a position having the highest priority is to be projected on the front-most surface.
Thus, supposing that a moving body A moves from the position of a past image A1 to the position of the current image A6 by moving diagonally from the right backward direction to the left forward of the screen surface 1 while subsequently passing through the positions of images A2, A3, A4 and A5. There will be an order of higher priority which corresponds to the positions Z=Z.sub.A6, Z.sub.A5, Z.sub.A4, Z.sub.A3, Z.sub.A2, Z.sub.A1, in the order beginning with images nearest to the screen surface 1, and there will be residual images A6, A5, A4, A3, A2, A1 between the past images A1 through A5 and the current image A6 as shown in the equation: EQU Z.sub.A6 &gt;Z.sub.A5 &gt;Z.sub.A4 &gt;Z.sub.A3 &gt;Z.sub.A2 &gt;Z.sub.A1 ( 1)
As a consequence, when it comes to the residual images A11 through A15 that correspond to the past images A1 through A5, as residual images at an infinite point Z=.infin., the part of the image A1 which is not overlapped by the past image A2 located at a position nearer to the screen surface is retained as a residual image A11, while the image part of the past image A2 which is not covered by the past image A3 located at a position nearer to the screen surface 1 is retained as a residual image A12, the part of past image A3 which is not covered by the past image A4 located at a position nearer to the screen surface 1 is retained as a residual image A13, the part of past image A4 that is not covered by past image A5 located at a position nearer to the screen is retained as a residual image A14, and an image A5 located at the position nearest to the screen surface 1 out of all the past images is retained as a residual image A15 as a whole.
On the other hand, it is thought that the current image A6 of the moving body A is located at a position of Z=Z.sub.A6 in the Z direction which is greater in the priority order in which the images are produced on the screen surface 1 than the residual images A15 through A11.
In addition, since the current image A6, and residual images A15, A14, A13, A12, A11 are images which go back into the past in a consecutive order, the residual images A5, A4, . . . A1 on the screen surface 1 are displayed so that the tone thereof becomes thinner while the current image A6 on the screen surface 1 has the thickest tone through a consecutive key processing of keying levels using keying levels K6 through K1 that correspond to the sizes up to the past points, as is shown in the following equation: EQU K6(=1)&gt;K5&gt;K4&gt;K3&gt;K2&gt;K1&gt;K0 (2)
consecutively from K6 (=1) as a key signal KEY, as shown in FIG. 1(B).
When such image processing is performed, the moving body A moves from the position of the image A1 to the position of the image A6 as described above so that the current image A6 and the part of the residual image A15 which is not covered by the image A6 are displayed together on the right side of the current image A6 on the screen surface 1. At the same time, the residual images A14, A13, A12 and A11 are displayed on the right side of the image A6 with the result that an image can be displayed on the screen surface 1 such that the moving image A moves in XYZ space while retaining a residual image thereof.
In the case where an image of a fixed plate B is assembled which is diagonally arranged in the Z direction with respect to the screen surface 1 so as to shield the moving track of such moving body A so that an image is obtained such that the moving body A having residual images A15 through A11 moves forward by passing through the fixed plate B from the backward direction, a higher priority is assigned to images located at a position nearer to the screen surface 1, with respect to the display priority order of the image A6, the residual images A15 through A11, and the fixed plate B, in the same manner as described with respect to the equation (1) on the basis of the position in the Z direction.
In such a conventional structure, as shown in FIG. 2(A), when the moving body A moves to a position of an image A3 which intersects the fixed plate B by passing through the position of the image A2 from the position of the past image A1, the moving body A is located at the position of the current image A3 (Z=Z.sub.A3) while the residual images A12 and A11 of the past images A2 and A1 are located at the position of Z=.infin.. At the same time, with respect to the keying signal KEY, as shown in FIG. 2(B), the largest keying level is assigned to the current image A3 in the order of the keying levels K3 (=1), K2 and K1 in the order of the current image A3 of the moving body A, the residual image A12, and the residual image A11 as shown in the equation: EQU K3(=1)&gt;K2&gt;K1&gt;0 (3)
At the same time, the low keying levels K2 and K1 are subsequently assigned to the residual images A12 and A11.
In this manner, the image displayed on the screen surface 1 at the timing at which the image A3 intersects the image on the fixed surface B is displayed on the screen surface 1 as an image B111 which is diagonally arranged on the basis the perspective method, as shown in FIG. 3, whereas the image A131, the part out of the current image A3 of the moving body A which projects forward of the fixed plate B, is displayed in the keying level K3 (=1) and the image part A132, the part out of the current image A3 of the moving body A which is covered by the fixed plate B, is not produced. At the same time, the image part A131 which is not covered by the fixed plate B is displayed in the tone of the keying level K3 (=1).
On the other hand, with respect to the residual image A12 of the past image A2, the image part A121, which is not covered by the fixed plate B or the current moving body A, is displayed in the tone of the keying level K2 whereas the image part A122, which is covered by the current image A3 of the moving body A, is not produced on the screen surface 1.
In the same manner, out of the past image A1 of the moving body A the part A111, which is not covered by the past image A2, is displayed on the screen surface 1 in the tone of the keying level K1 whereas the image part A112, which is covered by the image part A2, is not produced on the screen surface 1.
In this manner, the image on the screen surface 1 at the timing at which part of the image of the moving body A passes through the image on the fixed plate B is displayed so that images A2 and A3 are expanded gradually in a thicker tones which gradually come closer from the image A1 located at a distant position in the Z direction with respect to the screen surface 1. At the same time, only part A131, located forward of the fixed plate B, and an image part A133, located at an uncovered position at the rear of the fixed plate B, are displayed so that an image can be harmoniously displayed on the screen surface 1 which represents a state in which the moving body A partially passes through the fixed plate B while tailing along the residual image parts A121 and A111 of the residual images A12 and A11.
However, in accordance with the conventional method, there is a problem in that out of the residual images, the residual images A14 and A15 of the past images A5 and A4, which are located in front of the fixed plate B, cannot be produced as shown in FIG. 4 when the moving body A moves to a position where the fixed plate B is not covered after the moving body A completely passes through the fixed plate B.
Incidentally, since the priority of the residual images A14 and A15 of the images A5 and A4 in front of the fixed plate B is given to a position at an infinite point (Z=.infin.), the parts A153 and A141, located at positions that cover the image of the fixed plate B in the front of the fixed plate B, are covered on the contrary by the image B111 when the parts A153 and A141 are produced on the screen surface 1.
Thus, there is a problem in that only a special effect image which gives an sense of awkwardness can be generated, in that the residual image parts A153, A141 and A132 thereof, which should be displayed in the front by passing through the image B111 of the fixed plate B and the image B111 of the fixed plate B, cannot be produced in front of the image B111 of the fixed plate B despite the fact that it is desired that an image can be obtained such that the moving body A passes through the image of the fixed plate B while retaining the residual images A15, A14, A13, A12 and A11.